Hi Valerie,
CTE matching comes from the 'old' ceramic leadless chip carrier days.
Exact matching is not a practical idea, because
(1) for active components with power dissipation, it is better that the
component has a CTE about 2 ppm/C [depends on power dissipated] higher than the
PCB;
(2) all of these materials have CTE's in ranges; thus you get the ~CTE +/-
1ppm/C;
(3) how small the delta-CTE needs to be depends on the the attachment method
[balls (diameter), columns, compliant leads], and your reliability goals;
(4) what affects SJ reliability is the effective CTE; thus not all layers
have to be the same.
Second-best advice, get a copy of IPC-D-279; best advice, of course, is
consult me.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com


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